Micro–Nano Water Film Enabled High-Performance Interfacial Solar Evaporation

Highlights Micro–nano water film enhanced interfacial solar evaporator enables a high evaporation rate of 2.18 kg m−2 h−1 under 1 sun. An outdoor device with an enhanced condensation design demonstrates a high water production rate of 15.9–19.4 kg kW−1 h−1 m−2. A multi-objective predictive model is established to assess outdoor water production performance. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01191-6.

Before the solar evaporation, the PPy sponge was put on the traditional interfacial solar evaporator for 12 h to achieve the water transfer balance.The wet PPy sponge was placed in a cavity constituted of thermal insulation foam.Only the top of the PPy sponge was exposed to the air.The water supply to the PPy sponge in the solar evaporation experiments was controlled by a micro syringe pump (Longer, LSP01-3A, China).The mass change of the wet PPy sponge and cavity after solar irradiation was used to calculate the evaporation rate.The changes in surface morphology of the PPy sponge were recorded by an optical microscope (Nikon DS-Fi2, Japan).

S1.5 Outdoor Experiments
The ventilation fan was purchased from Xingyao Solar Technology (Dongguan, Guangdong, China), with a maximum working voltage of 12 V and a maximum working current of 2 A. The condenser was purchased from Yuliang Electronic Technology (Huizhou, Guangdong, China), with a maximum working voltage of DC 12 V and a maximum working current of 0.6 A. The ventilation speed and condensing temperature can be adjusted according to the needs of users.The whole device was powered by a solar panel that was purchased from Sikade Technology (Nanjing, Jiangsu, China).The long-term outdoor experiments were conducted in Hangzhou (China) for 160 days and 40 days were selected to analyze.Before long-term outdoor experiments, we first compared the water production rate of this device in different operation mode, taking days with similar weather conditions for analysis.The operating parameters were same as the long-term outdoor experiments.The weather conditions were shown in Table S6.

S2 Note S1: Heat Loss Calculation
The absorbed solar energy by the evaporator Q solar was calculated by Eq. (S1) where A was the area of the absorber, and q solar was the total energy of the incident light.
The heat loss was mainly composed of radiation loss, convection loss and conduction loss.
The radiation loss Q rad was calculated by Eq. (S2) where A eff was the area of the absorber (3.5×3.5 cm), ε was the emissivity (here about 0.78), and σ was the Boltzmann's constant (5.67×10 -8 W m -2 K -4 ), T 1 was the steady-state average temperature of the absorber (K), and T a was the ambient temperature (K).
The convective loss Q conv was calculated by Eq. ( S3) where A eff was the area of the absorber, h eff was the convective heat transfer coefficient (5 W m -2 K -1 ), T 1 was the steady-state average temperature of the absorber (K), and T a was the ambient temperature (K).
The conduction loss Q cond was calculated by Eq. ( S4) where C is the specific heat capacity of water (4.2 J g -1 K -1 ), m is the mass of water in the bottom tank (g), △T was the temperature change of water in the bottom tank (K) within a certain irradiation time.
The temperature parameters we used to calculate the heat loss were as follows: The heat loss normalized based on the solar flux was as follows: The Level Set interface automatically set up the equations for the convection of the interface.The transport of the fluid interface separating the two phases was given by Eq. (S5): where φ defined 0 in the air and defined 1 in the water; u was velocity field in fluid; γ was the mobility;  was the interface thickness parameter.
In the Phase Field interface, the two-phase flow dynamics was governed by a Cahn-Hilliard equation.The equation tracked a diffuse interface separating the immiscible phases.The diffuse interface was defined as the region where the dimensionless phase field variable went from -1 to 1.When solved in COMSOL Multiphysics, the Cahn-Hilliard equation was split up into two equations: where  was the mixing energy density.The Navier-Stokes equations described the transport of mass and momentum for fluids of constant density.In order to account for capillary effects, it was crucial to include surface tension in the model.The Navier-Stokes equations were then: where ρ was density of fluid; p was the pressure of fluid; I was turbulence intensity; μ was dynamic viscosity; F was external force acting on the fluid.
The capillary filling in the different interface was controlled by the different surface tension and water contact angle.

S3.2 COMSOL Simulation When Designing the Outdoor Device
A transient-state model was established to study the temperature field and humidity field of the outdoor device by the commercial software COMSOL Multiphysics [S2].
The outdoor device can be simplified as a 2D model as follows: Vapor flow in outdoor device was governed by turbulence equations where ρ was density of fluid; u was velocity field in fluid; p was the pressure of fluid; I was turbulence intensity; μ T was turbulent viscosity coefficient; μ was dynamic viscosity; was the temperature of fluid; F was external force acting on the fluid.
Turbulent viscosity coefficient (μ T ) was calculated by Eq.S12 where Cμ was model coefficients; k was the turbulence kinetic energy; σ ε was the Prandtl (Pr) number corresponding to the turbulent dissipation ε.
Heat transfer process in outdoor device was determined by the thermal equation where ρ was the density; C p was the specific heat capacity; T was the temperature; t was time; k was the thermal conductivity; q was the heat flux.The properties of the working fluid were derived from the COMSOL material library.
Humidity change process in outdoor device was described by the Eq.(S15) where w(φ w ) was the humidity (%); ρ g and ρ l were the density of the vapor and water (kg m -3 ); u g and u l were the velocity of the vapor and water (m s -1 ) w v was the mass fraction of vapor (%); D was the diffusion coefficient (m 2 s -1 ).
Before establishing the governing equation of the condenser, we made the following assumptions: 1) Humid air was a Newtonian fluid composed of dry air and water vapor.The physical parameters of the fluid were constant.2) The contact thermal resistance of the liquid film and the condenser plate was neglected.The influence of ambient thermal radiation and viscous dissipation was also neglected.
The governing equation for the condensation side of the condenser was shown as follows: 1. Continuity equation: Where α a and α w were the volume fraction of humid air and condensed water; was the quality source term.

Momentum Conservation Equation:
Where p was the static pressure; ρ and μ were the density and dynamic viscosity; G was momentum source term;  was surface tension; κ w was the curvature of the gas-liquid interface.

Energy conservation equation:
Where h was the fluid specific enthalpy; λ was the thermal conductivity; C p was specific heat capacity; S T was the energy source term.
The composition change at the condensing side was described by Eq. (S23) and Eq.(S24) where W a v and W a a w ere the mass fraction of dry air and water vapor in humid air.

Fig. S3
Fig. S3 The formation process of PDA on the surface of the PDMS surface [S3, S4] (a) The hydrolysis of APTES; (b) The formation of PDA on PDMS surface

Fig. S11
Fig. S11The digital photos of the salt-dissolution process on the PPy sponge. 2 g of salt were sprinkled on the surface of the PPy sponge working in the different-salinity brine.After 20 minutes, the salt completely disappeared, which indicated that the PPy sponge had good salt transfer performance Fig. S15 (a) The schematic diagram of the 3D interfacial solar evaporators containing the PPy sponge; (b) The evaporation rate of the 3D interfacial solar evaporators containing the PPy sponge with different height

Table S1
The TOC, COD, and TDS value of the original seawater and condensed water

Table S2
The evaporation rate of the interfacial solar evaporator based on different PPy and PDA coated sponges under 1 sun

Table S4
Weather conditions when measuring the water production rate of different samples

Table S5
The water production rate (kg m -2 ) of the different samples

Table S6
Weather conditions when operating the device in different operating modes

Table S7
The weather conditions for 7 cities we selected